[PVE-User] PBS : is dirty-bitmap really accurate ?

[Fabian Grünbichler]

On July 23, 2020 11:00 am, Jorge Boncompte wrote:
> El 23/7/20 a las 8:53, Daniel Berteaud escribió:
>> ----- Le 23 Juil 20, à 8:43, Fabian Grünbichler f.gruenbichler at proxmox.com a écrit :
>> 
>>> possibly you haven't upgraded to pve-qemu-kvm 5.0-11 (or your VM hasn't
>>> been restarted yet since the upgrade):
>>>
>>> https://git.proxmox.com/?p=pve-qemu.git;a=commit;h=f257cc05f4fbf772cad3231021b3ce7587127a1b
>> 
>> I'm running pve-qemu-kvm 5.0.0-11, and all the implied VM have been either (cold) rebooted, or migrated.
>> 
>>>
>>> the bitmap has a granularity of 4MB, so depending on the activity inside
>>> you can see quite a bit of amplification. also writing and then
>>> zeroing/reverting again to the old content would leave a mark in the
>>> bitmap without permanently changing the contents.
>>>
>> 
>> Yes, I'd expect some amplification, but not that much. For my Zabbix server, it's nearly canceling all the benefit of using a dirty bitmap.
>> One thing I've noted, is that I get expected values at least for one guest, running PfSense (where I get ~150MB of dirty blocks each days). Most of my other VM are Linux, I'll check if it could be related to the atime update or something
> 
> 	Hi, does the dirty-bitmap take somehow into account block discarding
> and zeroing? Because the other thing I would look for in this case is
> for a fstrim firing every day.

also a possible candidate. trim/discard of course changes the blocks and 
thus dirties the bitmap.